Bug 814251 - [email/IMAP] implement IMAP pipelining of FETCH requests to reduce latency. r=asuth, r=lightsofapollo, a=blocking-b2g=tef+

Land in gaia:
https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/176
Merge pull request #9189 from asutherland/imap-parallel-fetch

Bug 814251 - [email/IMAP] implement IMAP pipelining of FETCH requests to reduce latency. r=asuth, r=lightsofapollo, a=blocking-b2g=tef+